Uh... how do I do that, exactly?

I don't know how to add it, but here's a logo good day. https://commons.wikimedia.org/wiki/File:Mediawikis_on_the_loose.svg

Well, it's more of a sign, really. But I'm sure you could find a way to use it as a logo if you wanted.

In T87736#1364858, @Aklapper wrote:

What is a "Phabricator project logo" exactly? Or where would I be able to see that?

I think https://phabricator.wikimedia.org/project/profile/1005/ to see it, https://phabricator.wikimedia.org/project/picture/1005/ to change it.